The stratum corneum, located on the outermost layer of skin, has a barrier function that protects the body from contact and infiltration of foreign matter such as bacteria and hazardous substances, while also maintaining skin in a healthy state by preventing transpiration of moisture from the body. When the barrier function is impaired, transepidermal water loss (TEWL) from the skin surface increases, thus lowering the relative moisture content in stratum corneum. Reduction of moisture content in stratum corneum, resulting in an irregular texture of dermatoglyphs composed of sulci cutis and cristae cutis, creates a dry, roughened skin condition (Sone, T. et al., Koshokaishi, Vol. 15 No. 2. pp. 60-65 (1991)).
The intercellular lipids in the stratum corneum, which play an important role in the barrier function, are composed mainly of ceramides, free fatty acids and cholesterol, and they are known to form a lamellar structure (Bouwstra J A, et al., Acta Derm Venereol Suppl (Stockh). 2000 208: pp. 23-30; Bouwstra J A, et al., Int J Cosmet Sci. 2008 Oct. 30(5): p. 388). For development of cosmetics, it is necessary to examine the interaction between stratum corneum intercellular lipids and cosmetic materials beforehand to avoid impairing the barrier function of the skin, and to screen for effective materials.
For examination of the effects of cosmetic materials on skin barrier function, usually a biological sample is exposed to the candidate material and then its intercellular lipid structure is subjected to X-ray analysis, electron spin resonance (ESR), differential scanning calorimetry (DSC), infrared ray analysis (IR) or the like. Using a biological sample allows the necessary data to be obtained which can be linked directly to product information.
However, because of individual differences in biological samples, the stratum corneum intercellular lipids obtained from the samples also have different structures that depend on the particular sample. Furthermore, complex procedures are required to obtain specific stratum corneum intercellular lipids from animals, and measuring methods are limited in light of animal protection.
For development of new cosmetics, therefore, it has been necessary to employ convenient and highly reproducible indicators that do not rely on biological samples, in order to determine what effects the substances have on stratum corneum intercellular lipids.
Surfactants are among the components included in skin care products and the like. Surfactants penetrate the stratum corneum and are absorbed into the keratin of cornified cells, mixing with intercellular lipids (Friberg, S. E. et al., Colloids Surf. 1988, 30, pp. 1-12; Rhein, L. D. Ibid. 1997, 48, pp. 253-274). Surfactants are also known to remove cutaneous lipids such as fatty acids, fatty acid glycerides and cholesteryl esters, thus incurring damage to the skin even if the removed lipids are minimal (Fulmer, A. W. et al., J. Invest. Dermatol. 1986, 86, pp. 598-602; Denda, M. et al., Arch. Dermatol. Res. 1994, 286, pp. 41-46; Ronald, R. W. et al., J. Invest. Dermatol. 1999, 113, pp. 960-966; Lopez, O. et al. Bioch. et Biophy Acta 2000, 1508, pp. 196-209; Ebba, B. et al., Inter J. Pharma. 2000, 195, pp. 189-195).
The effects of surfactants on stratum corneum intercellular lipids have been an object of interest in the past in connection with the skin barrier function (Harada, K. et al, J. Invest. Dermatol. 1992, 99, pp. 278-282; Lavrijsen, A. P. M. et al., J. Invest. Dermatol. 1995, 105, pp. 619-624), and as regards changes in lipid composition related to various cutaneous symptoms (Holleran, W. M. et al, J. Lipid Res. 1991, 32, pp. 1151-1158; Murata, Y. et al., J. Invest. Dermatol. 1996, 106, pp. 1242-1249; Ponec, M. et al, J. Invest. Dermatol. 1997, 109, pp. 348-355). Nevertheless, the action processes and mechanisms are still poorly understood.